The present disclosure relates to a method for bonding substrates, and particularly, to a method of bonding substrates while minimizing stress induced by a mismatch in the coefficients of thermal expansion (CTE's) between the substrates during a cool down step, and an apparatus for effecting the same.
Substrate warp poses a challenge for the attachment of a semiconductor chip to a packaging substrate using flip-chip technology. A large warp in magnitude at reflow is a cause for unreliable C4 bonding, leading to potential non-wets or unequal C4 solder heights. In addition, a large sample to sample variation (sigma) of warp is a problem during bond and assembly even for substrates having a nominal mean warp. The thermal warp, i.e., the change in warp with temperature, is another undesirable problem for bond and assembly. A large thermal warp means that the packaging substrate changes shape during the critical cool down period after the reflow of the solder balls. Such change in the shape of the substrate could lead to defects in the solder ball joints such as hot tears.
One solution to the warp problem is the use of a mechanical constraint in the form of a temporary glass carrier (TGC) throughout the bonding process, including the entire duration of the cool down period. In this example, the packaging substrate is attached, with an adhesive, to a thick flat glass plate under pressure such that the inherent absolute warp of the substrate is flattened out. Because of the mechanical constraint of the glass carrier, the absolute and thermal warps are reduced close to zero, and so is the sample to sample variation (sigma) of this warp. However, modeling studies have shown that the stresses and shear strains in the C4 balls and back-end-of-line (BEOL) metallurgy are increased by constraining the free movement in the Z-direction of the packaging substrate during the cool down process. The difference in the coefficients of thermal expansion (CTE's) between the packaging substrate and the semiconductor chip causes a differential contraction between the two during cooling. This results in the build-up of shear stresses in the C4 balls and the semiconductor chip.
The shear stresses in the C4 balls can be naturally minimized by the bending of the bonded assembly including the semiconductor chip and the packaging substrate if the bonded assembly is free to bend. If the free bending of the bonded assembly including the semiconductor chip and the packaging substrate is restricted by use of the TGC, the reduction of stresses and strains cannot be achieved. Limiting the free bending of the bonded assembly can thus lead to an effective increase in failures in the C4 balls and BEOL metallurgy.
A factor that can mitigate the increase in stresses and strains, while using a TGC, is the potential to constrain the X-Y expansion of the substrate and thereby reduce the effective CTE difference between the die and substrate. Modeling studies have shown that the CTE of the constrained packaging substrate has to be reduced by nearly 40% for the stresses to be lowered to the stresses for a packaging substrate that is free to bend. Measurements on actual TGC mounted substrates have shown that such a large reduction in CTE is very difficult to achieve, and is highly dependent on the adhesive being used. Soft adhesives allow slippage between the substrate and the glass, and do not achieve a sufficient reduction in CTE. Harder adhesives can reduce the CTE more, but result in either permanently deforming the substrate or breaking the glass.
The TGC also has the drawback of requiring an adhesive to attach the packaging substrate to the glass. The adhesive contributes to contamination of a ball grid array (BGA) or land grid array (LGA) side of the substrate. Due to this phenomenon, the TGC technique cannot be used with LGA substrates where contamination is not tolerable. The removal of the packaging substrate from the TGC is also an issue. The technique of shearing a bonded package off the TGC is stressful to the packaging substrate. Through substrate vias in the packaging substrate are particularly vulnerable to damage during this process.